The oxygen affinity of hemoglobin (Hb) is significantly influenced by several allosteric effectors including 2,3-diphosphoglycerate (2,3-DPG), protons, chloride, and carbon dioxide. Several studies have characterized how these allosteric effectors modulate the reactivity of hemoglobin. The carbamino hemoglobin (HbNHCOO-) has been a difficult molecule to isolate; thus, the structure/function relationship of the CO2 binding to hemoglobin is not well understood. This study focuses on the thermodynamics of the CO2 binding of hemoglobin as a function of chloride. CO2 binds to the -NH2- terminal valine groups of the four chains of hemoglobin, with the beta-chains having a 3 fold higher affinity than the corresponding alpha-chains. NMR studies and the use of modified hemoglobins have shown that inorganic anions, specifically chloride, also bind at the same site (Val 1 -Arg141) and at several other sites (Lys82 [DPG site], His146 -Asp94). With microcalorimetry, we have determined the dependence of CO2 binding to hemoglobin as a function of chloride concentration. A drop-off in the enthalpy of the CO2-deoxy Hb interaction begins at 0.01M chloride and continues downward at high concentrations of chloride (1.00M). This result is consistent with the fact that CO2 and chloride are competitive allosteric effectors binding to the same site(2) or nearby site(s) and with the dissociation of the hemoglobin from tetramers to dimers at salt concentrations over 0.2M.